1. Field of the Invention
This invention relates to the field of battery systems, and particularly to rechargeable battery systems for mobile computers.
2. Description of the Related Art
Many electronic products include rechargeable batteries which enable them to operate without connection to an AC power source. The status of the battery is often of critical importance to the product""s user, as the product can operate only as long as the battery retains some useful life. For example, mobile computers, which are powered by a removable battery pack when not connected to an AC power source, typically provide a battery status screen to a user which includes an estimation of the remaining life for the battery housed within the pack.
One parameter which is often of interest is the charge level on the battery. Battery capacity in ampere-hours is typically calculated by measuring current flow in to and out of the battery, and integrating the current over time. Some recent mobile computers have embedded a microcontroller or complex finite state machine (FSM) integrated circuit inside the battery pack as a sub-system of the mobile computer. Using this device in combination with a number of sensors, voltage, temperature, current and various other variables are measured with circuitry housed within the battery pack itself. This results in a very complex and expensive battery pack. Nevertheless, this approach has been adopted as an industry standard by the Smart Battery System Implementers Forum (SBS-IF).
A block diagram of a battery system for a mobile computer which complies with the SBS-IF specification is shown in FIG. 1. A battery pack 10 includes a rechargeable battery and the associated sensors and circuitry mentioned above. Current is provided to and drawn from the pack via a line 12, and the pack is connected to a common point via a line 14.
When connected to an AC power source, an AC/DC converter 18 provides a DC supply voltage for the battery system via a line 19. When the DC supply voltage is present, it powers the system host 20 (which includes a mobile computer""s microprocessor and associated circuitry), typically via one or more DC/DC converters 22. A power detection circuit 23 detects the presence or absence of the DC supply voltage; when present, a switch S1 is operated such that power is provided to a battery charger 24, which in turn provides charging current to battery pack 10. When the DC supply voltage is absent, switch S1 connects battery pack 10 to DC/DC converters 22, so that system host 20 is powered by the rechargeable battery.
Communications between battery pack 10, system host 20, and battery charger 24 are handled with a serial bus referred to as an xe2x80x9cSMBusxe2x80x9d 26, which complies with the requirements of the SBS-IF. The system also requires that battery pack 10 provide a xe2x80x9csafety signalxe2x80x9d 28 to charger 24, to prevent overcharging.
Various conventional implementations of battery pack 10 are shown in FIGS. 2a-2d. Each battery pack includes the battery itself 30, a current sense resistor 32, a pair of FET switches 34 and 36, and circuitry 38. The FET switches are controlled by circuitry 38 to prevent either overcharging or over-discharging the battery 30. In each figure, the current through the sense resistor is designated as IS, and the current required to power circuitry 38 is designated as IC; IS, is determined by measuring the voltage across resistor 32 and dividing by its resistance. The current into or out of the battery pack is designated as IBP.
To accurately measure IBP, it is preferable to calibrate circuitry 38. Ideally, this requires the ability to measure the voltage across sense resistor 38 with no current flowing in it, to determine how much zero signal offset is in circuitry 38. However, when pack 10 is configured as shown in FIG. 2a, circuitry 38 always puts a small current drain (IC) on the battery. Even when IBP equals zero, IS=xe2x88x92IC and as IS flows through resistor 32, a zero-current condition cannot be achieved; thus, some current measurement inaccuracy is inevitable with this approach.
In FIG. 2b, the arrangement of battery 30 and current sense resistor 32 is changed. Here, the current IC required by circuit 38 does not pass through sense resistor 32, enabling the voltage across sense resistor 32 when IS is zero to be measured. Now, however, the IC current drain is never accounted for, thereby introducing a different standard error in the system.
Yet another arrangement is shown in FIG. 2c, in which current sense resistor 32 is referenced to ground. However, with resistor 32 connected between ground and battery 30, IS cannot be made zero, and thus the system cannot be calibrated for zero offset.
In the arrangement shown in FIG. 2d, a separate xe2x80x9cbattery groundxe2x80x9d 40 separate from system common (14) is employed, which allows the system to measure zero current. Here, however, IC is not measured and continually drains the battery. Furthermore, the lack of a common ground between battery and system induces a ground shift which is proportional to IS. For a large current load, this ground shift significantly reduces the noise margin of digital signals between system host 20 and circuitry 38.
A rechargeable battery measurement and calibration system is presented which overcomes the problems found in the prior art approach described above. The current measurement and battery status intelligence is moved outside of the battery pack, resulting in a system which has both higher accuracy and lower cost than prior art systems.
In accordance with the present invention, the battery pack is greatly simplified: the current sensing element is moved outside of the battery pack, the FET switches can be eliminated, and the battery status intelligence is moved from the battery pack to the system host; these steps significantly reduce the cost, complexity, and power consumption of the battery pack. The system host controls the measurement and calibration of the battery system; the host can command a zero current flow through the current sensing element, enabling the acquisition of accurate calibration data. In a preferred embodiment, calibration values are determined under both zero-current and non-zero current conditions, enabling linearity errors that might otherwise be present in the current measurements to be reduced.
The present system complies with the SBS-IF specification for Smart Battery systems, and is well-suited for use with mobile computers.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.